1. Field of the Invention
The present invention relates to electrical machines such as motors and generators, and more particularly to angular position and velocity estimation of permanent magnet (PM) synchronous machines.
2. Description of Related Art
PM machines can be classified based on the flux distribution or back-electromagnetic force (EMF) waveform as square-wave brushless dc machines (BLDCM) and sine-wave or synchronous machines (PMSM). The BLDCM utilizes a trapezoidal back-EMF waveform and supplied by rectangular current waveforms with conduction duration of 120°. The commutation is based on the rotor position with 60° resolution typically provided by Hall sensors. The PMSM utilizes sinusoidal back-EMF and supplied by sinusoidal current waveforms properly phased with respect to the back-EMF. The rotor position information is typically provided by resolvers or encoders.
Most of sensorless motor controllers utilize position estimation techniques for both BLDCM and PMSM using back-EMF. The initial rotor position information at standstill is not available using back-EMF methods. The possible initial starting methods can be classified based on the following:
Starting from predetermined rotor position established by proper feeding;
Open-loop start up; and
Estimation of the rotor position at zero speed based on state observers utilizing machine parameters.
See, M. Tursini, R. Petrella, F. Parasiliti, “Initial Rotor Position Estimation Method for PM Motors”, IEEE Transactions On Industy Applications, Vol. 39, No. 6, November/December 2003, pp. 1630-1640, which is incorporated by reference herein in its entirety.
The above techniques may not guarantee good accuracy in rotor position estimation at zero and low speed. A combination method utilizes a resolver at zero and low speed and a sensorless algorithm based on extended rotor flux at medium and high speed. See U.S. Pat. No. 7,072,790 to Hu et al., which is incorporated by reference herein in its entirety. The reason to use sensorless algorithms at medium and high speed is to reduce negative effect of non-ideal resolver characteristics such as an amplitude imbalance, imperfect quadrature, and inductive harmonics on the current controller and motoring torque.
U.S. Pat. No. 5,329,195 to Horber et al., which is incorporated by reference herein in its entirety, describes utilizing a PM motor with the stator pole elements wound with two coils. One coil on each pole element includes a sensor winding, and the other includes a power winding. As a result, all pole elements of the stator are utilized both to produce rotor drive torque and also to generate electrical signals that provide accurate rotor position information. This approach is useful for small size PM machines because of elimination of a somewhat similar sized resolver. However, for the larger size PM machines this approach considerably penalizes the power density of the machine, and, therefore, may not be attractive in applications such as aerospace or ground vehicles.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for systems and methods that allow for improved performance such as reliability, complexity, and cost. There also remains a need in the art for such systems and methods that are easy to make and use. The present invention provides a solution for these problems.